There is a need for a radiation detector which can provide an immediate indication of the existance of a low LET radiation field such as gamma rays or of microwaves. The detector should preferably be compact, be sensitive enough to detect radiation levels that are regarded as hazardous to man, give a reading or indication of such a level in an easily interpretable form while the device is being exposed, require no external source of power (such as A.C. supplies or batteries), be reusable so that many measurements can be made on different occasions and have the capability of storing the totality of readings from many measurements within the detector. Such a detector would find application in various areas of work including research, industrial, medical, environmental, military and civil defence. No existing gamma or microwave detector can meet these specifications.
Existing microwave detectors all require external power supplies for their operation and are therefore unsuitable in this context. A brief review of microwave detectors can be found in reference.sup.1.
Although an enormous number of devices have been used for the detection of gamma rays, the bulk of the common gamma ray detectors operate on three basic principles;
(1) they use electrical power to amplify or collect the radiation-induced events in order to make them detectable, e.g. gaseous counters, scintillation counters and semi-conductor detectors.sup.2 ; PA0 (2) they use radiation-induced change to a substance which is subsequently processed to yield an indication of the radiation exposure, e.g. thermoluminescent dosimeters.sup.3, photographic emulsions.sup.4, detector based on stimulated electron emission.sup.5, silicon diodes.sup.4, and radiation effects on glass.sup.4 or liquid chemicals.sup.6 ; PA0 (3) radiation induced change in a subtance or device which can be observed directly, e.g. pocket ionization chambers.sup.7, radiochromatic detectors.sup.8 and electrets.sup.9.
Since detectors in category 1 use electrical power, they are unsuitable for the above purposes. Since detectors in category 2 do not give an immediate indication of a radiation field, they need not be considered pertinent. Since gamma ray detectors in category 3 bear some common features with the desired detector, their principles and limitations will be discussed in greater detail.